This invention relates to a method of stabilizing fumed silica with respect to its retention of surface area. More specifically, the invention relates to a method of reducing the detrimental surface-area decay of fumed silica after it has been produced, wherein the silica contains an appreciable amount of chemically or physically bound fluoride.
Typically, fluoride-containing fumed silica is prepared by the flame hydrolysis of silicon tetrafluoride and contains about two to about four percent chemically and/or physically bound fluoride by weight. Such fumed silica is unstable in regard to the retention of its surface area under ordinary storage conditions, unless it has first been defluorinated. It is known that the surface area of fumed silica, which contains more than about 1.5% fluoride by weight, decreases rapidly and detrimentally under ordinary conditions. In a two-week storage period the surface area of such a fumed-silica product may decrease to as low as forty percent or less of its original value. It is also known that by defluorinating fumed silica, the same becomes increasingly stable under ordinary conditions and will not substantially change when the fluoride content is reduced to a level of about 400 to about 2100 parts per million by weight. However, it would be beneficial to be able to store high-fluoride-containing fumed silica with as little loss of surface area as possible, for example (1) when the defluorinator section of a plant is inoperative, and (2) when high-fluoride-containing silica is the desired end-product.